1. Field of the Invention
The present invention relates to a semiconductor display device (hereinafter referred to as display device), specifically, an active matrix display device having a thin film transistor that is formed on an insulator. More specifically, the invention relates to an active matrix liquid crystal display device that uses a digital signal as a video signal. The invention also relates to a portable information device employing this display device. Specific examples of the portable information device include a cellular phone, a PDA (Personal Digital Assistants), a portable personal computer, a portable navigation system, and an electronic book each comprised of the active matrix liquid crystal display device.
2. Description of the Related Art
Display devices having a semiconductor thin film formed on an insulator, a glass substrate, in particular, have gained a distinct popularity in recent years, and active matrix display devices employing a thin film transistors (hereinafter referred to as TFT) are especially popular among those display devices. Any of the active matrix display devices employing a TFT has from several ten thousands of TFTs to several millions of TFTs arranged into matrix and controls electric charges of pixels to display an image.
A technique that is being developed lately relates to a polysilicon TFT for simultaneously forming a pixel TFT and a driving circuit TFT. The pixel TFT is a TFT constituting a pixel, and the driving circuit TFT is a TFT constituting a driving circuit that is provided in the periphery of a pixel portion. The technique is a great contribution to reduction in size and reduction in power consumption of the liquid crystal display devices. Owing to the development of this technique, the liquid crystal display devices are becoming indispensable devices for, e.g., display units of mobile machines, which lately find their application in increasingly larger fields.
FIG. 13 shows a schematic diagram of an ordinary liquid crystal display device driven by a digital method. A pixel portion 1308 is placed in the center. Above the pixel portion, a source signal line driving circuit 1301 is arranged to control source signal lines. The source signal line driving circuit 1301 has shift register circuits 1303, first latch circuits 1304, second latch circuits 1305, D/A converter circuits (D/A converters (also called DAC)) 1306, analog switches 1307, etc. Gate signal line driving circuits 1302 for controlling gate signal lines are arranged to the left and right of the pixel portion. Although the gate signal line driving circuits 1302 are provided on both sides of the pixel portion in FIG. 13, only one gate signal line driving circuit may be provided to the left or right of the pixel portion. However, it is desirable to place the gate signal line driving circuit on each side of the pixel portion from the viewpoint of driving efficiency and driving reliability.
The source signal line driving circuit 1301 has a structure as the one shown in FIG. 14. The driving circuit shown in FIG. 14 as an example is a source signal line driving circuit with a horizontal resolution of 1024 pixels for 3 bit digital gray scale signals. The driving circuit includes shift register circuits (SR) 1401, first latch circuits (LAT1) 1402, second latch circuits (LAT2) 1403, D/A converter circuits (D/A) 1404, etc. Though not shown in FIG. 14, the driving circuit may have a buffer circuit, a level shifter circuit and the like if necessary.
Referring to FIGS. 13 and 14, the operation of the device will be explained briefly. First, clock signals (S-CLK, S-CLKb) and start pulses (S-SP) are inputted to the shift register circuits 1303 (denoted by SR in FIG. 14) and pulses are outputted sequentially. The pulses are then inputted to the first latch circuits 1304 (denoted by LAT1 in FIG. 14) so that digital signals (digital data) also inputted to the first latch circuits 1304 are held therein respectively. Here, D1 is the most significant bit (MSB) whereas D3 is the least significant bit (LSB). When the first latch circuits 1304 complete holding digital signals corresponding to one horizontal period, the digital signals held in the first latch circuits 1304 are transferred to the second latch circuits 1305 (denoted by LAT2 in FIG. 14) all at once in response to input of latch signals (latch pulses) during the retrace period.
Thereafter, the shift register circuits 1303 again operates to start holding digital signals corresponding to the next one horizontal period. At the same time, the digital signals held in the second latch circuits 1305 are converted into analog signals by the D/A converters 1306 (denoted by D/A in FIG. 14). The analog signals are written in pixels through source signal lines. An image is displayed by repeating this operation.
Now, a portable information device employing the above conventional liquid crystal display device will be described.
The description of the portable information device is given taking as an example a portable information terminal. FIG. 34 shows a block diagram of a conventional portable information terminal. The portable information terminal is intended to provide a user with desired information in accordance with the user's needs. The information to be provided includes data stored in memory devices (such as a DRAM 1509 and a flash memory 1510) in the portable information terminal, data stored in a memory card 1503 that is to be inserted to the portable information terminal, data obtained by connecting the portable information terminal to external equipment through an external interface port 1505, and like other data. The information is processed by a CPU 1506 upon receiving command inputted by the user via a pen touch tablet 1501 so that a liquid crystal display device 1513 displays the information.
Specifically, signals inputted through the pen touch tablet 1501 are detected by a detector circuit 1502 and then inputted to a tablet interface 1518. The inputted signals are processed by the tablet interface 1518 and the processed signals are inputted to a video signal input circuit 1507 and other circuits. The CPU 1506 processes necessary data, and the processed data is converted into image data based on an image format that is stored in a VRAM 1511. The image data is sent to an LCD controller 1512, which generates signals for driving the liquid crystal display device 1513. The display device is thus driven to display the information.
A cellular phone is taken as another example to describe the portable information device. FIG. 35 shows a block diagram of a conventional cellular phone. The cellular phone is composed of a transmission/reception circuit 1615 for transmitting and receiving radio wave, an audio processing circuit 1602 for processing signals received, a speaker 1614, a microphone 1608, a keyboard 1601 for inputting data, a keyboard interface 1618 for processing signals inputted through the keyboard 1601, etc.
Upon receiving command inputted by a user through the keyboard, a CPU 1606 processes information so that a liquid crystal display device 1613 displays the information. The information may be data stored in memory devices (such as a DRAM 1609 and a flash memory 1610), data stored in a memory card 1603 that is to be inserted to the cellular phone, data obtained by connecting the cellular phone to external equipment through an external interface port 1605, and like other data.
Specifically, signals inputted through the keyboard 1601 are processed by a keyboard interface 1618 and the processed signals are inputted to video signal processing circuit 1607 and other circuits. The CPU 1606 processes necessary data and the processed data is converted into image data on the basis of an image format stored in a VRAM (Video RAM) 1611. The image data is sent to an LCD controller 1612, which generates signals for driving the liquid crystal display device 1613. The display device is thus driven to display the information.
An example of the structure of the transmission/reception circuit 1615 is shown in FIG. 26.
The transmission/reception circuit 1615 includes an antenna 2662, filters 2663, 2667, 2668, 2672, and 2676, a switch 2664, amplifiers 2665, 2666, and 2677, a first frequency converter circuit 2669, a second frequency converter circuit 2673, a frequency converter circuit 2671, oscillation circuits 2670 and 2674, an AC/DC converter 2675, a data demodulation circuit 2678, and a data modulation circuit 2679.
In a general active matrix liquid crystal display device, screen display is updated about sixty times for every second in order to display animation smoothly. In other words, it is necessary to supply digital signals for every new frame and the signals have to be written in pixels each time. Even when the image to be displayed is a still image, the same signals have to be kept supplied for every new frame and an external circuit, a driving circuit and the like have to process the same digital signals repeatedly and continuously.
An alternative method is to write digital signals of the still image in an external memory circuit once and then supply the digital signals from the external memory circuit to the liquid crystal display device each time a new frame is started. However, the alternative method is not different from the above method in that the external memory circuit and the driving circuit of the display device are required continuing to operate.
In the conventional portable information device also, data of the same image have to be sent to the display device incorporated in the portable information device sixty times for every second in order to display any image on the display device, even if it is a still image. To explain this referring to the drawings, the circuits surrounded by the dotted lines in FIG. 34 must continue to operate as long as the image is being displayed (the circuits are: the video signal processing circuit 1507 in the CPU 1506; the VRAM 1511; the LCD controller 1512; the source signal line driving circuit and the gate signal line driving circuit of the liquid crystal display device 1513; the pen touch tablet 1501; the detector circuit 1502; and the tablet interface 1518). In the case of FIG. 35, the circuits surrounded by the dotted lines in FIG. 35 must continue to operate as long as the image is being displayed (the circuits are: the video signal processing circuit 1607 in the CPU 1606; the VRAM 1611; the LCD controller 1612; the source signal line driving circuit and the gate signal line driving circuit of the liquid crystal display device 1613; the keyboard 1601; and the keyboard interface 1618).
Passive matrix display devices have only a small number pixels, and some of them can stop operation of their VRAM during a still image is displayed by incorporating memory circuits in their driving ICs or controllers. However, incorporating a memory circuit in a driving or a controller is unpractical for a display device that uses a large number of pixels, such as an active matrix liquid crystal display device, from the viewpoint of chip size. Many circuits thus have to continue operating in a portable information device of prior art even when a still image is displayed, thereby forming an obstacle to reduction in power consumption.
Reduction in power consumption is greatly demanded in mobile machines. Despite the fact that mobile machines are used mostly in a still image mode, driving circuits of the mobile machines continue to operate during still image display as described above. Therefore, reducing power consumption is hindered.